In one type of running casing into a well or drilling with casing, the string of casing being made up is suspended in the well by a spider at the rig floor. The spider has hydraulically actuated slips. A pipe gripper mounts to the top drive, the pipe gripper having hydraulically actuated gripping members that grip an additional joint of casing to be made up to the string of casing. The operator rotates the top drive, which rotates the gripping members to make up the additional joint of casing with the string of casing. The operator then lifts the top drive, which raises the entire string of casing. The operator releases the spider slips and lowers the entire string of casing until the upper end of the newly added joint of casing is near the rig floor. The operator releases the gripper and picks up a new joint of casing to repeat the process.
A string of casing often contains hundreds of casing joints. Occasionally, an operator will inadvertently direct release fluid pressure to the spider slips when the gripper is in a released position. The weight of the string of casing may be adequate to prevent the release fluid pressure to release the slips; the slips normally have to move upward in order to release, and the casing string weight might prevent the slips from moving upward even if hydraulic fluid pressure is being applied. If the weight is not at that level, the slips will release and the entire string of casing will slide downward in the well. Depending on how far the casing slides, the results could be disastrous. Similarly, an operator might inadvertently direct release fluid pressure to the gripper when the spider slips are released.